Let the user change the dynamic linker used by ELF code.

	Let the user change the dynamic linker used by ELF code.
	* ld.h (args_type): Add new field interpreter.
	* lexsup.c (parse_args): Add dynamic-linker to longopts, and
	handle it.
	* ldmain.c (main): Initialize command_line.interpreter to NULL.
	* emultempl/elf32.em (gld${EMULATION_NAME}_before_allocation): Get
	the ELF backend to return the .interp section.  If
	command_line.interpreter is not NULL, set the contents of .interp
	to it.
	* ld.texinfo: Mention -dynamic-linker.